Deafness

Hearing impairment is the most common chronic handicap in developed countries like Singapore.

In the United States, only 9.7% of persons 65 years of age or older have normal hearing. It is also the most common industrial disease afflicting the economically active age group, not just the elderly.

To understand how we hear, we need to understand the various parts of the ear. It consists of an outer ear, consisting of the pinna and ear canal, which direct sound waves to the middle ear. The middle ear directs sound waves to the inner ear by the vibration of the eardrum and three tiny bones called the ossicles (the smallest bones in the human body).

The eardrum separates the external ear from the middle ear. The eardrum is air- and water-tight to protect the delicate structures of the middle and inner ear. The middle ear space where the ossicles are suspended is connected to the back of the nose by the auditory (eustachian) tube which opens intermittently when swallowing or yawning, allowing air pressure to equalize across the ear drum. This ability to equalize is affected by allergies and infections of the nose and sinuses. The negative ear pressure can cause an effusion with a decrease of the middle ear sound conduction.

The common conditions causing a conductive hearing loss are impacted wax, ear inflammation from infections, or negative pressure secondary to sino-nasal conditions as mentioned earlier. Sensorineural hearing loss exists when there is a lesion of the inner ear or related auditory nerves. The common conditions causing this kind of hearing loss are noise-induced deafness and deafness of aging.

Age-associated hearing loss was unknown in ancient times, probably because of an absence of excessive noise and a short life span. Deafness with age is generally viewed as the sum of insults to the aging auditory system.

The normal range of human hearing is 20 to 20,000Hz, with an upper limit of 10,000 by about the sixth decade. The range for human speech is 500 to 3000Hz. Exposure to excessive noise results in damage that impedes hearing at all frequencies above 4000Hz. An average daily noise exposure over 20 to 40 years to sound intensity of 90dBA (decibels on the A scale), as in some industrial settings, may result in a permanent pure-tone threshold shift beginning at 3000 to 4000Hz. We experience sounds ranging from 30dB (a soft whisper) to 140dB (a gunshot blast or a jet engine). Conversational speech is approximately 50 to 60dB.

As a rule, if you must shout to be heard, then usually, the ambient noise is hazardous to your hearing. Occupational noise is the most common source of excessive noise. However, personal leisure activities, such as listening to music and sporting events, are also of importance.

The poorer hearing at higher frequencies observed in men has generally been attributed to greater levels of exposure to occupational and recreational noise. In support of this explanation is the fact that no significant gender differences in high-frequency hearing have been noted in animal studies. Furthermore, in societies free of hazardous noise exposure, the hearing thresholds of elderly women and men are equivalent.

Early signs of hearing loss due to loud noise exposure include ringing in the ears and muffled sounds. Generally, it is painless, progressive and permanent. It can be prevented if hear protection is worn.

Hearing protection with earplugs is very effective for most noise levels. Rubber or plastic ear plugs fit into the ear canal. A snug fit is important, and so to have them customized-fitted is best for optimal comfort and protection.

Cotton plugs are not to be used, as they do not block any high frequency sounds and only a few low frequency ones. Earmuffs are the most effective protection against noise, since they cover the sound conducting bones around the ears as well as the ears themselves.

Deafness in children is commonly due to middle ear problems, secondary to sino-nasal disease, as mentioned earlier.

The long-standing negative pressure in the ear from a blocked auditory tube, secondary to thick mucus or from glands at the back of the nose (adenoids), would give rise to an effusion in the middle ear (glue ears). This presents a conductive hearing loss of moderately severe degree.

Babies with a low birth weight or born prematurely are prone to hearing loss (congenital). About 1 in 1000 children are born with congenital hearing loss. At least 1/3 and perhaps as much as 3/4 of these losses has a component that was inherited from one of the parents. Unfortunately, the diagnosis of hearing loss at birth usually is not made until the child reaches, on average, 2.5 years of age.

Children who are unable to hear well can have delays in emotional and personal development. It is not until 4 years of age that children develop an adult-like ability to discriminate clearly between speech and background sounds.

For babies, failure to startle or awaken to loud noises, to babble at about 3 months, or to begin to use words at 1 to 1.5 years’ of age may indicate hearing loss. Older children with a hearing loss may turn the radio or television to louder levels than usual. They may ask the parent to repeat words often or be inattentive at school.

Speech development should be accessed early, as normal speech will be delayed in the hearing-impaired child.

The parent’s assessment of the child’s hearing is particularly important. If the parent is suspicious of hearing loss, the doctor must be equally suspicious, until a complete audiometeric and otological evaluation has been completed.

In children who have uncorrected hearing handicaps, the ability to discriminate background noise from speech remains impaired, which can severely hinder a child’s ability to develop normal speech and language skills. Learning disabilities and emotional difficulties may then follow.

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